|Download and burn ultimate boot CD. Boot up with it and then run the various diagnostics programs on it to see if anything is highlighted as being problematic.|
Ultimate Boot CD can be downloaded from:
Memory test is one you certainly want to run and you may want to run several passes as your problem seems intermittent.
Does it blue screen then restart? If so you may want to disable restart on error so you can see the error message to track down the problem that caused it.
If you want to see what this error is, you will have to stop the reboot as Windows reboots too quickly for you to read what happened. You can disable the Auto-Reboot so you can read, and see the blue screen error.
Go to Start -> Control Panel -> System
Go to Advanced
Under the Startup and Recovery section, click Settings...
Under System Failure un-check "Automatically restart"
☺ When everything else fails, read the instructions.